This invention relates to a process for producing electrical resistance welded low alloy steel tubes, and especially for producing tubes provided with welds having improved toughness.
In the conventional method, a continuous steel strip is formed through a series of forming rolls into a tubular shape having a longitudinally extending seam, and the thus formed seam is then subjected to electrical resistance welding so as to produce a continuous tube called an "electrical resistance welded tube" (hereunder sometimes referred to as "ERW" tube). The ERW tube thus produced is then drawn to some extent with a sizer in order to adjust its roundness within a limited tolerance. Since the ERW tube may be highly efficiently manufactured and is of good quality, it has been widely used for a variety of applications as less expensive steel tube.
However, heat input for welding in high frequency electrical resistance welding is rather small, and the cooling rate of the welds is relatively high in comparison with that in other welding methods. Therefore, in a low alloy steel tube containing a relatively large amount of alloying elements, the area adjacent to the welds turns into a hardened structure upon cooling, resulting in degradation of its low temperature toughness and corrosion resistance.
In order to soften the hardened structure in the area adjacent to the welds, high frequency induction heating is applied immediately after the welding so as to heat the area adjacent to the welds to effect normalizing or annealing. This heat treatment is called "seam normalizing" or "seam annealing".
This heat treatment has the following advantages:
(1) Since the heat treatment can be carried out in a continuous production line for manufacturing tubes, it can be carried out efficiently without interrupting the continuous production of the ERW tubes;
(2) Heat treatment is applied only to a limited area adjacent to the welds, making the heat treatment less expensive; and
(3) Sizing with a sizer may be carried out after heat treatment, and an ERW tube can be manufactured within closer tolerances.
However, it also has the following disadvantages:
(1) When the distance between the surface of the tube to be heated and a high frequency induction heating coil is varied, the density of the induced current is also varied, resulting in fluctuation in heating temperatures. So it is quite difficult to precisely control the heating temperature, since fluctuation in the induced current density is unavoidable. Therefore, it is necessary to previously set the heating temperature at a level higher than the target heating temperature. In addition, when high frequency induction heating is employed, it is unavoidable that the current density of the induced current in the outer surface of the tube is higher than that in the inner surface of the tube, resulting in the heating of the outer surface to a temperature higher than the inner surface. The difference in temperature between the outer surface and inner surface sometimes reaches about 200.degree. C. or more. This means that the heating temperature of the outer surface inevitably comes to a temperature about 200.degree. C. higher than the target value. Such high temperatures sometimes cause the formation of coarse austenitic crystal grains. Improvement in toughness cannot be expected; and
(2) The welds have oxides of Si, Mn etc., which have been formed during heating and which extend in the radial direction of the tube when it is upset, i.e. when subjected to sizing. Such extended oxides in the radial direction impair toughness of the welds.
Therefore, it has been thought that ERW tubes have poor toughness in their welds, and they cannot be used in applications in which improved toughness is required.
A method in which the whole of the ERW tube is subjected to quenching and tempering has been proposed so as to improve the toughness of the tube. However, this method cannot produce ERW tubes within close tolerances and is much expensive because this method requires the additional heat treatments. These disadvantages offset the advantages which can be obtained by employing electrical resistance welding.